1. Field of the Invention
The present invention relates to an application of a display device used as a terminal of an information processing system, and more particularly to a display device which displays a plurality of images displayed at a fixed or semi-fixed scale such as a character, symbol or graphics having a predetermined code assigned thereto, at any desired position, and graphically displays image data of a bit map on a graphic memory.
2. Related Background Art
Of raster scan type graphic displays (hereinafter referred to as display devices), a bit map display device is primarily designed to display picture images such as line drawings and drawings and it is usually not suitable for displaying only a text.
FIG. 1 shows a prior art common display device 400. When an image is to be displayed on an image screen 406, a drawing unit (CPU and DMA controller) 401 writes image data into an image memory 402, and when character are to be displayed, it writes character codes into a character memory 403. The character codes are read from the character memory 403 in synchronism with the scan of the image screen 406 and they are converted to image data of the characters corresponding to the codes by a character font table 404. It is combined with the image memory 402 by a combine circuit 405 and sent to the image screen 406 for displaying the image of the characters. When as character is to be updated, the drawing unit 401 updates only that character code of the character memory 403 to facilitate the edition of the text. Such a prior art system is called a duplex display device.
In this system, since the memories are provided for the characters and the images, a control unit such as a memory read circuit has an image memory read circuit 412 for the image memory 402 and a control circuit 413 for a character memory 403. The drawing unit 401 for controlling the image screen also controls in duplex. Further, since the sizes and the display positions of the characters are preset, the application is limited.
FIG. 2 shows a prior art image-only display device 500 disclosed in Japanese Laid-Open Patent Application 3-23916 "Method of Writing Characters into an Image Memory". When a drawing unit 501 is to display an image (profile) of characters on an image screen 506, an image write-in unit 504 writes image data of the image of the characters corresponding to the character codes from a table of a character font memory 503 to an image memory 502. This system is less expensive than the duplex type display device of FIG. 1 by the elimination of the character memory, and the characters may be enlarged or reduced at any positions by processing the image data from the character font dot by dot. (This is hereinafter called a depiction type display device).
Naturally, image data of characters is written into the image memory 502 and the information as character codes has been lost. Accordingly, when a text is to be edited, the entire image data of the depicted character image must be controlled by the drawing unit. If the character image spreads over the entire image screen, the same memory capacity as that of the character memory 403 which is omitted from the duplex type display device 400 is required in order to control the image of the depicted characters. Further, where the update of the characters is frequently done, the time required to depict the characters accumulatively increases. Thus, it is not suitable to process a large amount of characters.
When an image and characters are to be displayed in mixture, the prior art duplex type display device needs large hardware but a freedom of character display (particularly, a freedom of display position) is limited. In the depiction type display device, the freedom of the character display position and the display size is improved, but when a text is to be edited with a high frequency of character update, the depiction time is relatively long and a burden to the depiction unit is heavy.